1. Field of the Invention
This invention relates to an air-fuel ratio sensor deterioration-detecting system for internal combustion engines, and more particularly to an air-fuel ratio sensor deterioration-detecting system of this kind, which detects deterioration of an air-fuel ratio sensor arranged in the exhaust system of the engine at a location downstream of a catalytic converter arranged therein.
2. Prior Art
To detect deterioration of an air-fuel ratio sensor arranged in the exhaust system of an internal combustion engine at a location downstream of a catalytic converter arranged therein, the following methods (1) to (3) are conventionally known:
(1) The air-fuel ratio of an air-fuel mixture supplied to the engine is forcedly changed from a rich value to a lean value or vice versa, by open-loop control of the air-fuel ratio (Japanese Laid-Open Patent Publication (Kokai) No. 5-256175). PA1 (2) When an output from the downstream air-fuel ratio sensor shows a leaner value than a predetermined reference value and at the same time a variation in the output continuously remains within a predetermined range over a first predetermined time period, the air-fuel ratio of the air-fuel mixture is set to a richer value than a stoichiometric air-fuel ratio. Thereafter, if the output from the downstream air-fuel ratio sensor still shows a leaner value than the predetermined reference value and at the same time the variation in the output value continuously remains within the predetermined range over a second predetermined time period, it is determined that the downstream air-fuel ratio sensor is deteriorated (Japanese Laid-Open Patent Publication (Kokai) No. 6-74074). PA1 (3) It is determined that the downstream air-fuel ratio sensor is deteriorated if the variation in the output from the downstream air-fuel ratio sensor continuously remains within a small range over a predetermined time period during execution of air-fuel ratio feedback control based on outputs from an air-fuel ratio sensor arranged upstream of the catalytic converter and the downstream air-fuel ratio sensor. PA1 the deterioration-determining means comprises increasing means for increasing a value of at least one of the first and second control parameters when a variation in the output from the second air-fuel ratio sensor is small during execution of the air-fuel ratio feedback control by the air-fuel ratio control means, and determining means for determining that the second air-fuel ratio sensor is deteriorated when the variation in the output from the second air-fuel ratio sensor is continuously small over a predetermined time period after the value of the at least one of the first and second control parameters has been increased. PA1 the deterioration-determining means comprises increasing means for increasing the value of the control parameter when a variation in the output from the second air-fuel ratio sensor is below a predetermined value during execution of the air-fuel ratio feedback control by the air-fuel ratio control means, and determining means for determining that the second air-fuel ratio sensor is deteriorated when the variation in the output from the second air-fuel ratio sensor continuously remains below the predetermined value over a predetermined time period after the value of the control parameter has been increased.
According to the above method (1), however, since the air-fuel ratio of the mixture is forcedly enriched or leaned by the open-loop control, the amount of generation of CO increases due to the enriching of the air-fuel ratio, or the amount of generation of NOx increases due to the leaning of the same.
Further, according to the above method (2) as well, the air-fuel ratio of the air-fuel mixture is forcedly enriched by the open-loop control, resulting in an increased amount of generation of CO, similarly to the method (1).
The method (3) does not cause degraded exhaust emission characteristics of the engine since the deterioration of the sensor is detected during execution of the air-fuel feedback control. However, the sensor output can sometimes remain close to a predetermined reference value for discriminating between rich and lean values, over the predetermined time period, even if the sensor is operating normally, resulting in a misjudgment that the sensor is deteriorated.